Posts Tagged ‘dislocation dynamics’

Deformation of metal micropillars

September 24, 2008

Sometime back, I linked to an article on the mechanisms of plasticity in Ni pillars under compression, and how the dislocation starvation in these systems leads to higher yield strengths. I understand that there is also a competing model which attributes the strengthening of micropillars to the internal pinning points and their effect on dislocation propagation.

In a recent paper in PNAS, C R Weinberger and W Cai try to resolve this controversy by trying to answer a more basic question, namely, how does a single dislocation, once nucleated move inside the micropillars.

Weinberger and Cai use computer simulations to study dislocation propagation in micropillars with FCC and BCC crystal structures and show that the crystal structure has a crucial role to play; here are their conclusions:

In summary, we discovered a new mechanism in which a single dislocation can multiply itself repeatedly in a BCC micropillar but not in an FCC micropillar. The mechanism is the combined result of both the surface-induced image stress and surface-dominated dislocation mobility at small scales. This discovery points to the necessity of different interpretations for the size-dependent yield stress on FCC and BCC micropillars. It also points to the importance of carefully accounting for surface effects on both the stress field and dislocation mobility in the dislocation dynamics modeling of plasticity at the microscale.

The paper, titled Surface-controlled dislocation multiplication in metal micropillars is available here ; and, here is the abstract:

Understanding the plasticity and strength of crystalline materials in terms of the dynamics of microscopic defects has been a goal of materials research in the last 70 years. The size-dependent yield stress observed in recent experiments of submicrometer metallic pillars provides a unique opportunity to test our theoretical models, allowing the predictions from defect dynamics simulations to be directly compared with mechanical strength measurements. Although depletion of dislocations from submicrometer face-centered-cubic (FCC) pillars provides a plausible explanation of the observed size-effect, we predict multiplication of dislocations in body-centered-cubic (BCC) pillars through a series of molecular dynamics and dislocation dynamics simulations. Under the combined effects from the image stress and dislocation core structure, a dislocation nucleated from the surface of a BCC pillar generates one or more dislocations moving in the opposite direction before it exits from the surface. The process is repeatable so that a single nucleation event is able to produce a much larger amount of plastic deformation than that in FCC pillars. This self-multiplication mechanism suggests a need for a different explanation of the size dependence of yield stress in FCC and BCC pillars.

Take a look!